In the past, as a large capacity external storage device which is used connected to a large-sized computer apparatus, a library apparatus has been known. A library apparatus is provided with medium storage racks which are provided with cells which hold cartridges in which magnetic tape media are contained and uses a conveyance robot which is provided inside the apparatus to convey cartridges to a tape drive so as to read or write data with respect to the tape media. In recent years, there have also been library apparatuses which are provided with pluralities of tape drives and library apparatuses which are provided with optical disk drives. Such library apparatuses are, for example, disclosed in Japanese Laid-Open Patent Publication No. 2006-127628.
A library apparatus has a fixed number of cartridges which it can store inside of it. Therefore, there are library apparatus which are designed to store cartridges in magazines and enable exchange of magazines so as change the cartridges which are contained. Further, in Japanese Laid-Open Patent Publication No. 2006-127628, two library apparatuses are stacked one on top of the other and a pass through mechanism is used to enable cartridges to be moved between the top level and bottom level library apparatuses.